The present invention relates to a superconducting coil apparatus wound with a cable-in-conduit superconductor made of superconducting wires housed in a metal conduit and cooled by coolant circulated in the metal conduit.
Methods for cooling superconducting coils are roughly classified into a pool boiling method and a forced flow cooling method. In the pool boiling method, the coil is directly immersed in the coolant. In the forced flow cooling method, the cable-in-conduit superconductor is wound to form a coil, and the coolant is forcibly circulated through internal passages formed in the conduit.
In case of the pool boiling method in which the coil is immersed in the coolant, it is required to provide a cryostat for housing the superconducting coil and coolant, as disclosed in Japanese Patent Unexamined Publication No. 98991/83 (JP-A-58-98991 ) published on June 13, 1983, for example. Further, the electrical insulation of the superconducting coil is influenced by the insulation of the coolant which is in contact with the outer surface of the superconducting wire to cool the wire. Accordingly, it is difficult to provide an apparatus having a high breakdown voltage.
In case of the forced flow cooling method, the cable-in-conduit superconductor itself serves as the coolant flow path. Accordingly, the cryostat for storing the coolant therein is not required. As a casing for enclosing the superconducting coil, the coolant, only a vacuum vessel with thermal insulation is required. Further, the breakdown voltage can be easily raised by selecting the insulation material because insulation depends on the surface of the conduit. In addition, the cooling performance is enhanced because the coolant is always flowing along the periphery of the superconducting wire located inside the conduit. In recent years, therefore, the forced flow cooling method is considered to be optimum to a superconducting coil such as a poloidal field coil for nuclear fusion reactor having a large-sized, complicated shape and producing high voltage. Thus the forced flow cooling method attracts attention from various fields for development.
A superconducting coil apparatus using the forced flow cooling-method is disclosed in Japanese Patent Unexamined Publication No. 14409/85 (JP-A-60-14409 ) published on Jan. 25, 1985, for example. The apparatus thus disclosed has various protective devices which are not concerned with the present invention. FIGS. 4 to 6 show principally the prior art apparatus using the forced flow cooling method as disclosed, but modified to show in detail only parts relating to the present invention by omitting the above described protective devices. The prior art apparatus will now be outlined by referring to FIGS. 4 to 6. Therefore, the structure shown in FIGS. 4 to 6 appears to be different from that illustrated in Japanese Patent Unexamined Publication No. 14409/85. However, it is to be understood that both apparatuses are the same in basic structure excepting the above described protective devices.
FIG. 4 is a sectional view of a forced flow cooling-type superconductor. The conductor as shown in FIG. 4 is used also in the present invention apparatus. A superconductor 1 is composed of a square-shaped pipe (conduit) 2 made of stainless steel and a number of superconducting wires 4 disposed in a coolant path 3 inside the pipe 2 along the path. By letting flow helium through the coolant path 3, the superconducting wires 4 are so cooled as to assume the superconducting state.
FIGS. 5 and 6 show a forced flow cooling-type superconducting coil 10 using the above described superconductor 1 and a typical coolant generating unit 17 disposed for the coil. Principal components are a circulation compressor 5, a housing vessel 9 for housing a liquid nitrogen tank 6, a liquid helium tank 7 and a heat exchanger 8 of countercurrent type, a cryostat 11 evacuated for housing a superconducting coil 10, coolant transfer pipes 12a and 12b for coupling the cryostat 11 to the housing vessel, current leads 14a and 14b respectively connected to ends 1a and 1b of the superconductor 1, and an electric power source 15. Cooling is conducted by a method described hereinafter. That is to say, helium forming the coolant is compressed by the circulation compressor 5 and led into the vessel 9 housing the heat exchanger. The helium is cooled to approximately 80.degree. K. in the liquid nitrogen tank 6 and exchanges heat with the return gas in the heat exchanger group 8. The helium is then cooled to approximately 5.degree. K. in the liquid helium tank 7 to become supercritical pressure helium. The supercritical pressure helium is supplied to the cryostat 11 through the helium transfer pipe 12a and combined in a terminal box 13 with the current lead 14a coming from the power source 15 to cool the superconducting coil 10. The return gas reenters the vessel 9 housing the heat exchanger through the return helium transfer pipe 12b. The return gas then undergoes J-T expansion in a Joule-Thomson valve 16 to be liquefied. The liquid helium is stored in the liquid helium tank 7. The gas evaporated here and the gas which is not liquefied return to the circulation compressor 5 through the return pipe while exchanging heat with the incoming gas. The above described process is repeated to cool the superconducting coil.
Drawbacks caused when such an apparatus is used to cool the superconducting coil will now be described. As evident from FIG. 6, the prior art apparatus is not especially equipped with means for preventing the intrusion of the heat from the current leads 14a and 14b. Only the thermal conduction of the circulating coolant is used. Accordingly, cooling is insufficient for heat intrusion caused by the thermal conduction from the external normal temperature section and the heat generation attendant upon the flowing current. Thus it takes a long time to cool the coil and the temperature of the coolant is raised. As a result, the superconducting state of the coil cooled in the forced flow mode becomes unstable.
Problems of heat generation caused by the current flowing through the current leads and temperature rise of the superconductor caused by the heat intruding from the outside through the current leads are present also in a superconducting coil apparatus using the pool boiling method. In order to prevent such problems, in the aforementioned Japanese Patent Unexamined Publication No. 98991/83, for example, the current leads are inserted into a tube and cooled by passing through the tube a vaporized gas of the liquid helium in the cryostat. However, this method cannot be applied to a superconducting coil apparatus of forced flow cooling method in which no cryostat is used.